The present invention relates to a control method and device of an engine which has a homogeneous-charge-compression-ignition (HCCI) combustion mode in which a premixed air-fuel mixture is compressed in a combustion chamber so that a self-ignition of the mixture is performed without requiring spark from a spark plug, and a minus valve overlap period during which an intake valve and an exhaust valve are both closed at a transitional time from an exhaust stroke to an intake stroke of the engine.
Conventionally, as disclosed in U.S. Pat. No. 6,425,367, the compression self-ignition gasoline internal combustion engine, in which a fuel injector to inject fuel directly into a combustion chamber is provided and the air-fuel mixture is compressed by a piston so that a self-ignition of the mixture is performed, is known. Herein, the minus valve overlap period during which an intake valve and an exhaust valve are both closed at a transitional time from an exhaust stroke to an intake stroke is provided, and a first fuel injection from a fuel injector is executed at a point after a closure timing of an exhaust valve and before the top dead center of an exhaust stroke during the minus valve overlap period and a second fuel injection from the fuel injector is executed at a point during an intake stroke.
According to the gasoline engine disclosed in the above-described publication, the combustion chamber is heated by the combusted gas (remaining gas) which has not been exhausted during the minus valve overlap period, and the above-described first fuel injection is executed under this high-temperature condition, so that the injected fuel can be reformed properly to improve the ignitability of the air-fuel mixture.
However, if the first fuel injection is executed at the above-described timing (after the closure timing of the exhaust valve and before the top dead center of the exhaust stoke of the engine) disclosed in the above-described publication in a range where the engine load is relatively high, there is a concern that the injected fuel may combust near at the timing of the top dead center of the exhaust stoke, thereby the combustion chamber may have a too-high temperature and a too-high pressure. This state would possibly cause a detonation (abnormal combustion), such as pre-ignition, which may be initiated by the above-described second fuel injection. This state would also prevent the induction of the sufficient amount of air into the combustion chamber during the intake stroke, so that there is a concern that the combustion temperature may increase so much that the NOx emission would deteriorate.